1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and a condenser microphone, and more specifically, to a semiconductor integrated circuit mounted on a condenser microphone, and a condenser microphone using the semiconductor integrated circuit.
2. Description of Related Art
Recently, in condenser microphones, the component mounting areas have been decreasing in accordance with the miniaturization of microphones. Because of this, it has been desired to decrease the number of components to be mounted and to integrate capacitors for cutting high frequencies. However, there is caused a problem that the ESD withstand voltage decreases by integrating the capacitors. In general, the withstand voltage of 8 kV is required in a microphone in ESD test (IEC61000-4-2). Thus, it is desired to integrate the capacitor while securing the ESD withstand voltage of the microphone.
FIG. 9 shows a configuration of a related example disclosed in Japanese Registered Utility Model No. 3081106 (Chin). In FIG. 9, a terminal A is a device input terminal having one end connected to a gate of an output transistor MN1 through resistors R1 and R2 and the other end connected to a diaphragm C3 of a condenser microphone 2. A terminal B is an output/power supply terminal that functions both as a power supply terminal and an output terminal. One end of the terminal B is connected to a drain of the output transistor MN1, and the other end of the terminal B is connected to a terminal E of the condenser microphone 2. A terminal E is connected to a power supply VDD through a load resistor RL. Further, the terminal E is connected to an output terminal VOUT of an AC signal.
The resistors R1 and R2 and diodes D1 and D2 in the device are protection circuits, and may be eliminated if they are not needed. Further, a resistor R3 is a pull-down resistor for determining an input impedance of the device, and is normally from several hundreds of MΩ to several tens of GΩ. A resistor R4 is a source resistor which is used for gain adjustment. The resistor R4 may be eliminated when it is not needed.
The voltage input to the diaphragm C3 of the condenser microphone 2 is input to the gate of the output transistor MN1. Then, after being subjected to current amplification, the voltage is output from the output terminal VOUT. Thus, the input signal is amplified and impedance conversion is performed. Between the terminal B and the drain of the output transistor MN1, capacitors C1 and C2 are provided as ground capacitors in order to remove the RF noise (800 MH to 2 GHz). These capacitors C1, C2 function as filters for the RF signal to decrease the RF noise.
In the related example disclosed in Chin, the capacitors C1 are C2 are integrated to realize miniaturization. In the related example, capacitances of 10 pF and 33 pF are employed as the capacitors C1 and C2. However, in general, it is difficult to increase the size of the capacitors integrated in the semiconductor. When the capacitances of 10 pF and 33 pF are employed, the chip size becomes large.
Although it is possible to decrease the chip with the capacitive element having large unit capacitance such as a gate capacitance, the capacitive element tends to be highly susceptible to ESD when the unit capacitance becomes larger, which means it is difficult to accomplish the desired ESD level. Integrating the capacitors for the miniaturization of the microphone and the ESD resistance conflict with each other, and it is difficult to realize both of them with the related circuit.